Inboards and cold weather?

[bsb]

I don't have any experience with inboards. My boat is FWC and i was wondering about the sea water side of the cooling system freezing after i flush it when it is below freezing out. There is a small amount of water that stays in the system from the sea water pump to the exhaust outlets. Is this something to be concerned about? If i don't flush the motor after i use it i don't think the sea water will freeze but fresh water will. Any advice or experience would greatly be appreciated. I have found that loosening the cover on the sea water pump will drain this excess water but is that necessary?